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ABSTRACT

Shales exhibit a wide range of textures, compositions, and mechanical properties, which are interlinked by their diagenetic history. During hydraulic fracturing of shales, the matrix is subjected to shear deformation, which may create microfractures and enhance hydrocarbon transport from nanoscale, organic matter (OM)-hosted pores to the larger, induced fracture network. To study the nanoscale response to shear deformation of shale pore systems with different diagenetic histories, we deformed shale samples from a formation in the Northern Rocky Mountains and the Eagle Ford Group in Texas, using confined compressive strength tests. N2 and CO2 adsorption were performed to quantify fracture effects on pore morphology including pore size distribution, porosity, surface area, and surface fractal dimensions. Most samples increased their gas adsorption quantity, pore volume, and surface area after failure. The surface fractal dimensions were less sensitive to shear deformation. Results show that varying nanometer-to-micron-scale fracture patterns are in part caused by contrasting rock fabrics that are preconditioned by their distinctive diagenetic histories. For example, fractures tend to propagate along the OM laminae, whereas others cut across OM grains and access OM pores. Other possible mechanisms for porosity increase include the deformation of relatively uncemented clay aggregates and contrasting amounts of intra-OM pores between samples. Thus, the mechanisms for syn-deformational porosity changes at the micro scale are highly dependent on diagenetic history, particularly the maturation of OM, and the cementation history relative to clay content.

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